Add adc_hpf() to allow control of ADC high pass filter.

unsigned short .adc_hpf(uint8_t bypass, uint8_t freeze);
unsigned short .adc_hpf(uint8_t bypass);
bypass & freeze as defined on page 34 of the SGTL5000 datasheet.
Merge pull request #15 from robsoles/master

Add adc_hpf() to allow control of ADC high pass filter.